1. Field of Invention
The present invention relates to compositions and methods for selectively removing polysilicon in preference to silicon dioxide and/or silicon nitride by chemical-mechanical polishing.
2. Description of Related Art
Chemical-mechanical polishing (CMP) is an enabling technology for the manufacturing of multi-level interconnects in integrated circuit (IC) fabrication. It is also becoming an enabling technology in the development of and manufacturing of micro-electro-mechanical devices (MEMS) and micro-opto-electro-mechanical devices (MOEMS). Like IC devices, multilevel structures have been widely used in MEMS devices (e.g., micro engines, pressure sensors, micro mirrors, micro lenses etc.). These devices contain moving parts within the multi-level structures, and planarity at each level becomes important. As layers of polysilicon and silicon dioxide are deposited and etched during successive steps in the device fabrication process, considerable uneven topography arises which imposes limitations in subsequent processes like patterning, deposition and etching. Specifically, to minimize the processing difficulties related to photoresist step coverage and depth of focus during photolithography, it is desirable to planarize each deposited layer using CMP.
An important feature of a MEMS multilevel structure that differentiates it from a typical IC, from a CMP perspective, is the large step height that needs to be worked with during MEMS fabrication. Structures on a single level in MEMS (polysilicon/silicon dioxide) typically have a step height of ≧5 microns, which is relatively large when compared to the ≦1 micron step height typical in IC fabrication. Thus, due to the large step height associated with the fabrication of MEMS devices, development of a robust CMP process faces several challenges. In addition, in several polishing steps during the device fabrication sequence, selective polishing of the top layer over the underlying layer is required. One such step involves polishing a few microns thick polysilicon film deposited in a silicon dioxide trench with or without silicon nitride. Hence, a slurry that selectively removes polysilicon over underlying silicon dioxide or silicon nitride is critical to prevent erosion of silicon dioxide and/or silicon nitride, which could be detrimental to subsequent fabrication steps.
The composition of the polishing slurry is one of the important factors in a CMP process. A typical CMP slurry includes abrasive particles and chemicals that serve as mechanical and chemical agents, respectively. Two of the important requirements for an acceptable slurry for the CMP of polysilicon in MEMS fabrication are: (1) a high (e.g., ≧200 nm/min) polish and/or planarization rate for the polysilicon film; and (2) a high polish rate selectivity (e.g., ≧50) of polysilicon to the underlying stop layer (e.g., silicon dioxide and/or silicon nitride). High polysilicon polish rates lead to short polishing times, whereas high polish rate selectivity minimize the erosion of the underlying material, thereby protecting the underlying devices. Hence, a CMP slurry and method that provide very high polysilicon polish rates and high polish rate selectivity for polysilicon with respect to underlying materials during MEMS device fabrication is highly desirable.